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All the effects of sedimentation, and the evidence we can use to interpret past conditions, come to nothing if the sediments are not preserved. Sediment remains unconsolidated for many years, and is often reworked and eroded away. Some, however, will remain relatively unscathed while it is buried beneath younger sediment deposited on top. The original sediment is, if you like, fossilised.
Cementation occurs as fluids - usually aqueous solutions - percolate through the pore spaces in the sediment. Chemicals precipitate from solution, filling the pore spaces and bonding the grains together. The sediment is cemented.
As more and more of this younger sediment builds up (the overburden), the pressure causes the original sediment to be compressed and reduced in volume. The grains are forced together and water in the pore spaces is squeezed out. The sediment is compacted.
A number of changes take place during lithification, for example
- the sediment is consolidated and becomes more competent
- the sediment is de-watered as a result of the pressure
- hydrocarbons and dissolved minerals migrate elsewhere
- porosity is reduced
There are implications here for the use of sedimentary rocks as aquifers and reservoir rocks for oil.
Geopix Study Guide "Lithification and Diagenesis"
- explains the terms involved in diagenesis and lithification;
- describes how cementation occurs, and describes the main types of cement;
- describes how compaction occurs;
- describes the origin of concretions;
- suggests how to answer exam questions on this topic.
The Lithification and Diagenesis Study Guide contains the following pages:
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To order this CD go to the Geopix Online Store